Abstract: Thermoelastic instability (TEI) during the synchronizing process of clutches is the main reason for hot spots on the frictional pair. On the basis of vehicular shift clutches, a three dimensional finite element model was built by Galerkin method for determining the critical speed of TEI, the growth rate of the perturbation and the temperature on the nodes, which were confirmed by the bench test. In the process, the growth rate of the temperature perturbation was calculated by an eigenvalue solution. Both theoretical and experimental results show that a strong inhomogeneous temperature field appears (its temperature fluctuation rises from 3% to 35% in 1 second), and hot spots appear along the circle of the mating disk on condition that the working speed exceeds the critical speed. Some structural and material properties, e.g., the thickness of the mating disk and frictional disk, the radius of the clutch and the ratio between inner and outer radius, the thermal conductivity and the elastic modulus of the frictional material, have significant effects on the unstable behavior of the clutch, but the effect of their heat-expanding coefficient is less.